This invention relates generally to control system for a hydraulic system, and more particularly, to a control system for a hydraulic system having a hydraulic transformer.
Hydraulic transformers are useful devices in a hydraulic circuit or system. A hydraulic transformer is a hydraulic power transmission and regulation device which is used in hydraulic systems or circuits. A hydraulic transformer provides pressure and flow energy transformations within the hydraulic circuit. Unlike valves, which only provide pressure reductions by throttling the flow through an orifice which incurs energy losses, the hydraulic transformer can provide an increase or decrease in pressure with corresponding increase or decrease in output flow. This is accomplished without incurring significant energy losses. Hydraulic transformers are typically used in conjunction with constant or known supply pressure as a source of power. The power source may be driven by any of a variety of prime movers such as a diesel engine, gasoline engine, piston or rotary engine, or an electric motor. The hydraulic transformers also need a hydraulic pumping device in conjunction with some type of pressure regulation system to provide the hydraulic transformers with a predetermined or constant supply pressure. This usually involves some other components such as hydraulic accumulators, pressure reducing valves, and variable displacement pumps with pressure compensation. In this manner, pump flow is adjusted to provide a constant known output pressure simultaneously with matching the output flow to the time varying demands of the hydraulic transformer connected to the hydraulic power source.
The functioning of the hydraulic transformer can be explained by an equivalent system consisting of a fixed or variable displacement motor connected to a fixed or variable displacement pump with at least one of these devices being variable or adjustable through some external means of control. The motor and pump can be two physically separate components interconnected with a shaft or can share the same pumping element. The hydraulic transformer may also have a port plate which used three fluid passages or ports. The displacements of the motor and the pump may be varied by changing the angle of the rotatable port plate. The pump and motor displacements can be related to each other and this relationship is a function of the angle of the port plate. The motor or inlet side of the hydraulic transformer can be connected to a constant pressure fluid power source possibly employing an accumulator or other means for maintaining constant supply pressure. The output or pump part of the hydraulic transformer is used to drive a hydraulic circuit, such as the propulsion, steering, or implement circuits found in an earthmoving machine. In essence, the hydraulic transformer is a three port device which transforms an input flow and pressure (i.e., power) into a different output flow and corresponding output pressure as a function of the pump and motor displacement ratio. The hydraulic transformer is capable of maintaining the same power level as at the input of the hydraulic transformer, except for mechanical losses in the hydraulic transformer. The third port, connected to the reference pressure point, provides the additional flow required at the output or bypasses the excess flow at the input, as is required by the transformation conditions. The transformation ratio between the input pressure or flow and the output pressure or flow of the hydraulic transformer depends on the effective displacement ratio between the input and the output which is controlled by the angle of the port plate within the hydraulic transformer.
In using a hydraulic transformer as a power transmission element in a hydraulic circuit it is important to require that the power output of the hydraulic transformer satisfy the demands of all the hydraulic circuits or loads connected to the hydraulic transformer. The hydraulic transformer also needs to provide stable and responsive performance under varying load conditions. When an accumulator is used at the input of the hydraulic transformer to provide a constant supply pressure, it is possible for the accumulator to bleed down, i.e., operate at reduced pressure, in order to maintain the required flow or instantaneous power. This occurs when the momentary power demand at the output exceeds what the energy is being supplied to the hydraulic transformer. Bleed down is a significant concern in the operation of a hydraulic transformer since the accumulator needs to be charged up to the standard system pressure prior to further system operations proceeding in a normal manner.
Another significant problem which arises with the use of a hydraulic transformer is due to the vary nature of its operation as a power transformation device. Depending on the size of the volumes and their effective compressibilities, sustained oscillations can be generated in the input and the output flow and pressure due to the interaction between the two volumes within the hydraulic transformer. Under extreme conditions, such oscillations can cause catastrophic component damage or failure. Additionally, such oscillations can cause operational difficulties in power transmission. Hence, such oscillations need to be eliminated to allow for acceptable functioning of the hydraulic transformer in most realistic applications. Further, providing against oscillations will permit the stable delivery of hydraulic power without excessive acoustical or noise generation within a hydraulic circuit.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention, a hydraulic transformer provides hydraulic pressure to a fluid actuator, a control system has a hydraulic system for providing hydraulic pressure to the hydraulic transformer, a controller connected to the hydraulic transformer, the controller for determining the input pressure provided to the hydraulic transformer and for controlling the operation of the hydraulic transformer based upon input pressure provided to the hydraulic transformer.
In another aspect of the present invention, a control system has a hydraulic system for providing hydraulic pressure to the hydraulic transformer, a controller connected to the hydraulic transformer, the controller for determining the output pressure provided to the fluid actuator from the hydraulic transformer and for controlling the operation of the hydraulic transformer based upon output pressure.